1. Field of the Invention
This invention relates to a zero-current switching DC power conversion process. And more specifically, resonant power converters using a series-resonant input stage to source AC voltage to an input power transformer via energy storage in the series-resonant capacitor. Wherein a compound frequency consisting of a forced sine wave oscillating frequency, Fo, couples to a resonant sine wave oscillating at its natural frequency, Fn. The two components combine to synthesize a composite, sine wave carrier-frequency providing a uni-polar and/or a bi-polar power supply transfer function to the power transmission loop.
2. Description of Prior Art
Sundry methods exist for converting an input level of DC power to another level for delivery to a prescribed load. Two conventional design methodologies in popular practice are generally classified as either: linear or switching mode. In view that either finds acceptance in certain application requirements neither enjoys the broad appeal of universal usage. Linear supplies exhibit low noise generation by virtue of internal power current remaining continuous while regulation is implemented by insertion of an artificial resistance exhibiting infinite resolution in series with the output load. While the variable resistance yields a low noise characteristic at the output terminals, objection is based on inefficient power transfer.
Switching supplies present a more efficient method for converting power from one voltage level to another. Power conversion in switching converters takes place by closing a power switch to couple the conversion circuit to an input power source during a portion of the transfer cycle. The power switch is opened during the remaining portion of the transfer cycle decoupling the power conversion circuit from the input power source and allow an energy charge, stored in the output integrating-filter inductance, to circulate through the output load. This method for converting power is very efficient since input current flows only during the time of power switch closure. Thus, input power is described in terms of duty cycle: Pin (pk) D. The principal drawback to this type of power conversion lies in the fact that switching action interrupts input current converting it into a train of discontinuous pulses. The resulting pulse train introduces a broadband frequency spectrum with a high-end, cut-off frequency, several orders of magnitude beyond the actual switching frequency.
Interruption of power current leads to circuit parasitics being bounded by a broadband frequency envelope. Transmission of frequency signals through the range in which parasitic L-C parameters resonate introduces ringing which becomes evident as noise on the output terminals. Suppression of this noise component resident on the output termination is a complex and tedious process that can impose serious penalty on conversion as well as volumetric efficiency.
Fortunately, 5V logic features a sufficiently wide noise threshold as to allow design procedures to become somewhat routine in their application. The same observation, however, does not necessarily apply toward evolving logic of lower voltage levels. Industrial trends toward 3V logic impose serious consideration with respect to tolerable noise envelopes. Further reduction of logic voltages to 1V, or fractional voltage levels, will require very high signal-to-noise ratios. In all probability, larger than can be accommodated by presently acceptable switching converter design disciplines.
An object of this invention is to describe a design process incorporating desired properties associated with either linear or switching power conversion design disciplines. Specifically, the low broadband noise envelope associated with linear power converter designs and the efficient, duty modulated, power transmission of high frequency switching topologies.
Another object of the invention is to define a design discipline in which an over-all DC to DC power conversion process incorporates an inner AC to DC power supply loop, wherein power supply input current is continuous throughout the entire power transmission cycle, and completely isolated from power switch transitions.
Another object of the invention is to illustrate capability of the inner power supply loop to accept any combination of half, or full-wave, rectifier/filter assemblies on the secondary windings of the isolation transformer for delivery of power to prescribed output loads.
Another object of the invention is to demonstrate a fly-back characteristic embedded in the resonant frequency, Fn, component which determines back-EMF impressed on the input power switch relative to design coefficient K selected for the composite carrier-frequency.
Another object of the invention is to illustrate a bi-polar power transmission process allowing non-polarized magnetic transformation and automatic re-set of the flux field in the magnetic core while using a single power switch referenced to the input power return bus.
Another object of the invention is to convey ready adaptability to synchronous rectification due to soft crossover of output and fly-back currents circulated in the transformer and rectifier/filter assembly.
The transfer functions described herein abide by standard industry definition for circuitry employed in power conversion applications. A power converter converts power from a DC source to a DC output. A power supply converts power from an AC source to a DC output. It is in this context that the power transfer function described herein embeds an inner power supply loop within an overall power converter transfer function. A practical circuit for achieving continuous AC current flow in a power transformer while responsive to variable switching for duty cycle control consists of a series-resonant circuit including an input DC voltage source; a resonating capacitor, a resonating inductor, a unidirectional conducting device and a power switch. The second stage, describing an AC voltage source. Is made up of a capacitor/transformer combination upon which a complex impedance, consisting of a reflected load component shunting the open-circuit inductance of the primary winding on the power transformer, acts as a load on the AC voltage source provided by the series-resonant energy storage capacitor. The load component acting on the capacitor introduces a time-constant that while working in concert with the angular velocity of the series-resonant input stage, serves to describe the sine wave frequency for the forced oscillation, Fo, component of the composite carrier-frequency.
The negative region of the AC voltage source waveform consists of a resonant frequency created by resonance between the voltage source capacitor and the open-circuit inductance of the primary winding on the power transformer. This frequency region introduces a resonant frequency component, Fn, to the composite carrier-frequency waveform. Power switch location in the series-resonant circuit places it outside the current loop formed by interactive energy circulation between the transformer""s primary winding and the voltage source capacitor. Therefore, power switch transition does not interfere with continuous current flow through the transformer primary winding providing source energy to the AC power supply inner-loop.
The transformer""s secondary winding acts as an intermediate hand-off to the current loop formed by the output integrating-filter for half-wave circuit configurations. In this instance, secondary winding current flows only during the power transfer stroke. Primary current, however, flows throughout the entire composite frequency cycle, with the negative voltage region providing the core flux re-set volt-seconds. Secondary winding current, however, remains continuous throughout its positive and negative region for full-wave circuit configurations. While primary winding properties remain the same for either half or full-wave rectification.
Design criteria inclusive of objects and brief general description above is further disclosed as a family of new resonant power converters based on primary-side resonance with zero-current-switching. The composite sinewave frequency applied to the primary winding of an input isolation transformer consists of a compound frequency whose constituent elements are predicated on a design coefficient K derived from the desired ratio of: Fn/Fo. The forced oscillation frequency, Fo, appearing as the positive transition in the composite waveform being somewhat dependent on the series-resonant input current source and the complex impedance impressed upon the capacitor/transformer loop by the open-circuit inductance of the isolation transformer acting in parallel with the reflected load resistance. The resonant frequency oscillation, Fn, appearing as the negative transition in the composite waveform is predicated wholly on the resonant elements in the parallel-resonant tank circuit, consisting of the resonant capacitor which terminates the series-resonant input current source and the transformer open-circuit inductance. Parallel-resonant circuitry is designed for maximum impedance in compliance with over-all dictates for minimal internal loss components to insure optimized transmission efficiency. The unloaded parallel-resonant circuit exhibits an inherent tendency for its internal impedance to migrate toward infinity due to regenerative properties characteristic of parallel-resonant tank circuitry.
The complex impedance impressed upon the parallel-resonant tank circuit, by the load resistance acting in parallel with the transformer""s open circuit inductance, shunts the parallel-resonant tank circuit""s inordinately high internal reactance in a manner in which the complex impedance being of a comparatively, low, ohmic value overwhelms the inordinately high value of the parallel-resonant tank circuit""s internal reactance. A resulting 90xc2x0 phase shift between voltage and current in the parallel-resonant tank circuit due to the quadrature relationship between the in-phase load and out-of-phase inductive components spreads the shunting impedance throughout the time-frame of the composite frequency Fo+Fn. Thereby stabilizing the entire power transmission carrier-frequency envelope.